In antenna measurements gating techniques are utilized to reduce the effects of echoic environments on the acquired antenna patterns, as for instance described in J. E. Hansen, Spherical Near-Field Antenna Measurements, London, United Kingdom: Institution of Engineering and Technology/Peter Peregrinus Ltd., 1988 and M. M. Leibfritz et al., “A Comparison of Software- and Hardware-Gating Techniques Applied to Near-Field Antenna Measurements”, Advances in Radio Science, Volume 5, pp. 43-48, 2007. Under ideal conditions the transmitting (TX) and receiving (RX) antenna would be positioned in free space or an absorptive box and only the line-of-sight path would contribute to the received signal. In practical measurements the environment surrounding the antennas is never ideal and multipath propagation corrupts the measurement signal. Due to the longer propagation distance, the non-line-of-sight (NLOS) signal components are delayed compared to the line-of-sight (LOS) signal and can thus be gated out in the time domain.
M. D. Blech et al., “Time-Domain Spherical Near-Field Antenna Measurement System Employing a Switched Continuous-Wave Hardware Gating Technique”, IEEE Transactions on Instrumentation and Measurement, vol. 59, no. 2, pp. 387-395, February 2010 discloses a time-domain spherical near-field antenna measurement system capable of gating out erroneous signal components, which arise due to multipath propagation in non-ideal anechoic chambers. The developed hardware (HW) gating technique evaluates a switched sinusoidal signal, which is synthesized by an application-specific pulse generator and acquired by either a real-time digitizing oscilloscope or an equivalent-time sampling oscilloscope. The measurement system presented in the above cited article of M. D. Blech et al. has been optimized for acquisition speed, dynamic range, and resolution. Its operating frequency range covers 1.5-8 GHz, and it is applicable to antennas exhibiting a typical 3-dB bandwidth in excess of 400 MHz.
In mm-wave antenna measurements conventional gating techniques like the so called hardware gating, employing RF-switches cannot be used as they suffer from a slow switching speed, a low isolation and a high attenuation. Application specific pulse generators using the concept presented in the above cited article of M. D. Blech et al. cannot be realized as digital building blocks are not available for high frequency ranges as used in mm-wave applications, in particular for frequencies above 1 GHz, preferably above 100 GHz.
Quasi-time domain measurements replacing a vector network analyzer (VNA) as described in K. Shibuya et al., “Compact and Inexpensive Continuous-Wave Subterahertz Imaging System With a Fiber-coupled Multimode Laser Diode”, Applied Physical Letters, Appl. Phys. Lett. 90(16), 161127, 2007 are very time consuming as the transfer function of the system needs to be measured for several hundred discrete frequencies over a wide bandwidth and for each of these frequencies there needs to be a variable delay, which usually is realized by a precise linear stage, which must be stepped through all the required positions. So, in total 2n measurements need to be carried out, where n is the number of frequencies, which must be measured in the frequency domain in order to achieve a certain temporal resolution. M. Scheller and M. Koch, “Terahertz Quasi Time Domain Spectroscopy”, Optics Express, vol. 17, no. 20, pp. 17723-17733, 2009 describes a method using a multi-mode laser diode, but this technique still suffers from a large number of steps carried out by a linear stage.